The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine and, particularly, to such an air-fuel ratio control apparatus capable of controlling the air-fuel ratio to a value for producing a leaner air-fuel mixture than a mixture at the stoichiometric air-fuel ratio when the engine is in a predetermined operating region so that the engine can operate with the thus controlled air-fuel ratio.
A type of air-fuel ratio control apparatuses in which control, known as "lean control", is employed, has been used in recent years. In lean control, the air-fuel ratio of the internal combustion engine is controlled to a value for producing a leaner air-fuel mixture than a mixture at the stoichiometric air-fuel ratio, that is, controlled to a value on the lean side of the stoichiometric value, with a view to reducing the level of fuel consumption and emission. In order to achieve the maximum possible effect for the reduction of fuel consumption and emission, such air-fuel ratio control is required to be able to control the air-fuel ratio to a value immediately behind a misfire limit of the air-fuel ratio. To meet the requirement, various methods for determining the misfire limit have been carried out. An example of such methods is Japanese Patent Unexamined Publication No. 60-122234, which discloses an air-fuel ratio control apparatus.
The air-fuel ratio control apparatus has an arrangement based on the fact that, when the condition of combustion in an internal combustion engine shifts from a normal region toward a limit beyond which the combustion condition enters into a misfire region, variations of the rotation of the engine increase. In the arrangement, a misfire limit is determined on the basis of such increased variations of engine rotation. Specifically, the speed of rotation of the engine is detected by a crank-angle sensor, and variations of engine rotation at a predetermined crank angle are sequentially calculated on the basis of the detected rotational speed. Standard deviations are calculated from the variations of engine rotation, and are each compared with a threshold value previously set in correspondence with a misfire limit. When the standard deviation is less than the threshold value, the combustion condition is determined to be still within a normal region, and an amount for correcting the air-fuel ratio is amended for correction to a value on the lean side. When the standard deviation is above the threshold value, it is determined that the misfire limit has been reached, and the air-fuel ratio correction amount is amended for correction to the rich side. The air-fuel ratio of the internal combustion engine is controlled in this way around a misfire limit. Since the air-fuel ratio corresponding to the misfire limit varies between various operating regions of the internal combustion engine, such as those concerning the number of revolutions per unit time of the engine and the amount of intake air, the disclosed air-fuel ratio control apparatus is arranged to set a threshold value for each of a plurality of operating regions.
Thus, the above air-fuel ratio control apparatus is arranged to set a threshold value for each operating region, and to effect air-fuel ratio control in accordance with a misfire limit appropriate to the current operating region. With this apparatus, however, no consideration is given to differences in the degree of variation of engine rotation between individual internal combustion engines. As a result, a misfire limit may not always be determined correctly.
That is, in an internal combustion engine, variations of engine rotation may be generated even within a normal combustion region employing, for instance, the stoichiometric air-fuel ratio. Such engine-rotation variations are caused by factors such as variations in the condition of combustion in the engine per se, detection errors of the crank-angle sensor, or errors of a CPU clock, and the variations assume values differing between individual internal combustion engines. With the above air-fuel ratio control apparatus, a standard deviation, which is calculated as the sum of an engine-rotation variation component caused by factors, such as above, and a component related to an increased risk of misfire, inevitably includes a fraction influenced by differences between individual internal combustion engines. Since a shift from a normal combustion region to a misfire limit causes a much smaller increase in engine-rotation variation than a complete shift from the same region into a misfire region, the influence of differences between individual internal combustion engines makes it impossible for a misfire limit to be correctly determined. As a result, the air-fuel ratio may be erroneously controlled, sometimes to a value on the lean side, resulting in misfire, and hence, impaired drivability, and in other cases, to a value on the rich side, making it impossible to achieve sufficient reduction of fuel consumption and emission.